Optical transmitter system

ABSTRACT

An optical transmitter system supplies a plurality of different wavelength optical radiation components to a transmission element which, in operation, is connected to the transmitter system for conveying the different wavelength radiation components to a respective receiver. The supply of the wavelength components is interrupted, preferably in response to a loss of the wavelength components at the respective receiver. A timer provides respective restart times which differ one to another for the wavelength radiation components. The supply of the wavelength components is restarted, following an interruption, at the respective restart times provided by the timer.

The invention relates to an optical transmitter system and moreespecially to an optical transmitter system for use in a wavelengthdivision multiplex (WDM) optical communications system.

In conventional optical communication systems, communications traffic iscommunicated between a transmitter and receiver by means of opticalradiation which is modulated with the communications traffic and whichis conveyed by optical waveguiding means, typically an optical fibre.Optical radiation in the context of this patent application is definedas electromagnetic radiation within a free-space wavelength range from560 nm to 2000 nm, although a free-space wavelength of substantially1550 nm is a preferred part of this range.

In operation of optical communications system an optical fibre cansuffer mechanical damage including breakage and it is desirable toprevent personnel in the vicinity of such a break being exposed to theescape of optical radiation, which may be of a level, which is hazardousto their eyesight or skin.

Two approaches to minimising this risk are: (i) to ensure, by design ofthe system, that the power level of the optical radiation carried by thefibre is always maintained below a “safe” level or (ii) to stop thesupply of optical radiation to the fibre when there is an indicationthat a break may have occurred in the fibre. In the case of the latterthe loss of the receipt of optical radiation at the receiver is taken asan indication of a potential fibre break. The latter method is oftentermed automatic laser shutdown (ALS). To re-start operation of thecommunications system, after ALS, it is known to re-start the laserafter a specified period of time, currently 100 seconds, and pulse itfor a short period of time. The duration of the pulse (typically twoseconds) is selected to be sufficiently short as to not cause injury.When the receiver detects such a pulse it communicates back to thetransmitter to recommence normal operation.

To increase the transmission capacity of the communications system it iscurrent practice to employ wavelength division multiplexing in which themodulated optical radiation comprises a plurality of radiationcomponents, termed wavelength channels, having mutually differentwavelength bands. Each wavelength channel is modulated with respectivecommunications traffic and all of the channels simultaneously conveyedover a single optical fibre. Systems with eight or more wavelengthchannels are often termed dense wavelength division multiplex (DWDM)systems.

For WDM systems the optical power conveyed by a fibre is the sum of theseparate component (wavelength channel) power levels and it isundesirable to limit this total power level to a safe level since thiswould unduly limit the span of the optical link between the transmitterand receiver. This is especially so for DWDM systems operating with manytens of wavelength channels and such systems consequently favour ALSarrangements.

In active WDM systems one of the wavelength channels, termed an OpticalSupervisory Channel (OSC), is reserved for controlling thecommunications system by passing control protocols between nodes. Theloss of OSC at the receiver is used as an indication of a break in afibre and the lasers for all the wavelength channels are accordinglyshut down. To re-activate the system OSC only is pulsed at 100-secondintervals for 2 seconds to test the optical fibre link. If the OSCreaches the receiver the lasers for the communications channels areaccordingly reactivated.

A problem arises however in passive WDM systems in which the wavelengthchannels are passively combined and each originates from optical sourcesthat operate in complete independence of one another. In passive systemsthere is no central control function for safely re-activating thesources and consequently it is preferred in such systems to limit thetotal optical power to a “safe” level.

The present invention has arisen in an endeavour to at least in partovercome this problem.

According to the invention, a transmitter system for supplying aplurality of different wavelength optical radiation components to atransmission element which, in operation, is connected to thetransmitter system for conveying the different wavelength radiationcomponent to a respective receiver, comprises: means for detecting aloss of a wavelength radiation component at the respective receiver; andmeans for interrupting the supply of the wavelength radiation componentin response to a detection of loss of the wavelength component at thereceiver; characterised by timing means capable of providing respectiverestart times which differ one to another for the wavelength radiationcomponents; and means for restarting the supply of the wavelengthradiation component, following an interruption, at the restart timeprovided by the timing means.

Detection of the loss of the wavelength radiation component at therespective receiver is taken as an indication that there may be a breakin the transmission element, or at least a problem in the source of thewavelength component and the supply of this radiation component isaccordingly interrupted by the transmitter system. Clearly the loss ofall of the radiation components at the respective receivers, which willresult when there is a break in the transmission element, will result inthe transmitter system interrupting the supply of all radiationcomponents thereby preventing accidental escape of radiation from thebreak. Since the transmitter system reactivates supply of the radiationcomponents at different restart times this ensures that even if there isa break when the system restarts only one, or only a few, of theradiation components will be present thereby limiting the power level ofany escaping radiation. Moreover since the source of each wavelengthcomponent is restarted at its own restart time, independent of any othersource, the present invention is particularly suited to a passive WDMsystem in which there is little or no central control function betweenthe sources.

In one arrangement, the timing means is capable of generating apseudo-random sequence of respective restart times for the differentwavelength radiation components.

Preferably, the timing means includes a timing unit at a source of eachdifferent-wavelength radiation component capable of generating arespective restart time. Preferably, the timing unit includes apseudo-random number generator and the seed number used in thepseudo-random number generator is unique to the source.

In an alternative arrangement, the timing means is capable of generatinga plurality of respective restart times based on the wavelength of theradiation component.

In one arrangement employing a seed number, the seed number used in thepseudo-random number generator includes the network address and portnumber of the source equipment.

In another arrangement employing a seed number, the seed number used inthe pseudo-random number generator includes a laser card serial numberof the source.

The invention also provides a method of operating an optical transmittersystem comprising: supplying a plurality of different wavelength opticalradiation components to a transmission element for transmission to arespective receiver; detecting a loss of a wavelength radiationcomponent at the respective receiver; interrupting the supply of thewavelength radiation component to the transmission element on adetection of loss of the wavelength radiation component at the receiver;supplying respective restart times which differ one to another for thedifferent-wavelength radiation components; and following aninterruption, restarting supply of the wavelength radiation component atthe respective restart time.

An optical transmitter system in accordance with the invention will nowbe described, by way of example only, with reference to FIG. 1 that is aschematic representation of a passive optical WDM transmitter/receiversystem in accordance with the invention. The transmitter/receiver systemprovides n bi-directional communications links with a correspondingtransmitter/receiver system located at an opposite end of thecommunication link.

Referring to the FIG. 1, the optical WDM transmitter/receiver systemincludes an output port 1 for connection to an output optical fibre 2capable of transmitting a WDM optical signal comprising a plurality n ofdifferent wavelength channels (λ₁ to λ_(n)), optical radiationcomponents. Typically the system would be configured for operation inC-band (i.e. a free-space wavelength of 1530 to 1565 nm) and have 32wavelength channels with wavelength spacing between channels of 0.8 nm.It will be appreciated that the present invention applies equally toother wavelength ranges of operation having differing numbers ofchannels/channel spacings.

The WDM signal is generated by a wavelength selective multiplexer 3which passively combines the individual wavelength channels (λ₁ toλ_(n)) from a respective optical fibres OF₁ to OF_(n). As is known thewavelength selective multiplexer 3 could typically comprise an arrayedwaveguide grating or cascaded dichroic filters. Each fibre OF₁ to OF_(n)is connected to respective client equipment 4, which transmits therespective wavelength channel.

The transmitter/receiver system further includes an input port 5 forconnection to an input optical fibre 6 for receiving a WDM signalcomprising the plurality of n wavelength channels. The received WDMsignal represents the communication link in an opposite direction. Apassive wavelength selective de-multiplexer 7 is connected to the inputport 5 and is operable to separate the constituent wavelength channelscomprising the WDM signal and place each on a respective optical fibreidentified as LF₁ to LF_(n). The Fibres LF₁ to LF_(n) are in turnconnected to a respective client equipment 4. The wavelength selectivede-multiplexer 7 could typically comprise an arrayed waveguide gratingor cascaded dichroic filters

In FIG. 1 there is shown only two client equipment 4 corresponding tobi-directional communications links that utilise wavelength channels λ₁and λ_(n) respectively. It will be appreciated that similar clientequipment 4 exists for the other wavelength channels though these arenot shown in the Figure for reasons of clarity. Each client equipmentcomprises a transmitter 8, transmitter controller 9, timing unit 10 andreceiver 11. The transmitter 8 is operable to generate the respectivewavelength channel and would comprise, for example, a semiconductorlaser whose output radiation is modulated with communications trafficusing an external optical modulator (e.g. A Mach-Zehnder opticalmodulator) connected to its output. It will be appreciated that inalternative configurations the laser can be modulated directly bymodulating the drive current to the laser with the communicationtraffic. The output of the transmitter 8 is coupled into the respectiveoptical fibre OF₁ to OF_(n). The timing unit 10 is operable to generatea respective restart time (t₁ to t_(n)) for its associated transmitter8. The transmitter controller 9 is for controlling operation thetransmitter 8 and as will be described is operable to shut downoperation of the transmitter (laser) in the event of a possible fibrebreak and to subsequently automatically restart the transmitter. Thereceiver 11 is for receiving and detecting communication trafficreceived on the respective optical fibre LF₁ to LF_(n).

In the operation of the transmitter/receiver system, each transmitter 8generates and transmits its respective wavelength channel along itsrespective optical fibre OF₁ to OF_(n) to the multiplexer 3 whichpassively combines the wavelength channels (optical radiationcomponents) to produce a WDM optical signal which is transmitted fromthe output port 1. The WDM optical signal is carried over the opticalfibre 2 to a corresponding transmitter/receiver system (not shown) at afar end of the fibre.

Further in the operation of the transmitter system, a WDM signal isreceived at the input port 5 and conveyed to the wavelengthde-multiplexer which passively separates the radiation components(wavelength channels) placing one on each of the optical fibres LF₁ toLF_(n) from which the wavelength channels pass to the receiver 11 in arespective client equipment 4. The received WDM signal represents theopposite direction of communication for the bidirectional communicationslink.

The possibility exists that, in the operation of the system, mechanicalbreakage could occur in either of the optical fibres 2, 6 connecting thetransmitter/receiver system to a corresponding transmitter/receiversystem at a far end. For ease of description, the operation of thesystem in relation to a break occurring in the optical fibre 2 will bedescribed though it will be appreciated that the system operates in alike manner for a break occurring in the optical fibre 6. Moreover inthe following description the transmitter/receiver equipment illustratedin FIG. 1 will be referred to as the near end system and thetransmitter/receiver at the far end referred to as the far end system.

The power level of the WDM signal being transmitted along the opticalfibre 2 is the sum of the power levels of the individual wavelengthchannels conveyed by the optical fibres OF₁ to OF_(n) and may be highenough to cause damage to, for example, the vision or skin of personnelin the vicinity of the break. As a safety measure, thetransmission/receiver system includes provision for treating a loss ofthe WDM channels at the far end system as an indication that amechanical break may have occurred in the optical fibre cable 2 and forswitching off the transmitters in the near end system in that event.Safety is assured in that, if there is a mechanical break in the opticalfibre 2, there is effectively no escape of laser-generated radiation,which would be at potentially hazardous power levels, from the break.

When a loss of the respective WDM channel is detected by the receiver 11in the far end client equipment 4, this being indicative of a possiblebreak in the fibre 2, the transmitter controller switches off itstransmitter 8. In turn the loss of the wavelength channel is detected bythe receiver 11 within near end client equipment 4 and the transmittercontroller 9 shuts switches off its transmitter 8. Clearly in the caseof an actual fibre break there will be a loss of all wavelength channelsat the far end and each transmitter will accordingly switch off itstransmitter.

In the operation of the system, following the switch-off of thetransmitters, on an indication that a mechanical break may have occurredin the optical fibre 2, the system recommences transmission after a setperiod has elapsed in case the break has been repaired. This is done foreach wavelength channel at a respective restart time t₁ to t_(n) whichdiffer one to another and which is generated by a respective timing unit10. Since the re-activation of the transmitters (i.e. those at the nearend) occurs at different restart times, the power transmitted into theoptical fibre cable 2 at restart is too low in level to pose a threat tosafety should it turn out that a mechanical break does exist in theoptical fibre 2. The sequence of interrupted transmission followed byrestart at respective restart times is repeated for as long as there isa detection of a break in the optical fibre 2 and, when a break is nolonger detected at, restart proceeds until normal full transmission isattained.

Typically, there are 100-second wait periods before transmission isrecommenced with restart attempts lasting of the order of 2 seconds. Assoon as the receiver 11 at the far end detects the presence of thewavelength channel, the transmitter controller 9 re-activates itstransmitter 8 and in turn this is detected at the near end by thereceiver 11. Upon receipt of the wavelength channel the transmittercontroller 9 switches the transmitter 8 to operate continuously.

Since each client equipment 4 includes a respective timing unit 10 thisenables the system to be safely re-activated, after a possible fibrebreak, without a need for any interaction between the client equipment4. In the embodiment illustrated each of the client equipment 4 operatein total in total independence of one another.

In a preferred implementation the timing units generate their respectiverestart time using a pseudo-random sequence. In such an arrangement thetiming unit includes a pseudo-random number generator and the seednumber used in the pseudo-random number generator is preferably uniqueto the transmitter.

The seed number used in the pseudo-random number generator can includethe network address and port number of the source equipment or,alternatively, the seed number used in the pseudo-random numbergenerator can include a transmitter (laser) card serial number oftransmitter.

There is a possibility that more than one transmitter will be restartedat the same time as each timing unit generates a pseudo-random restarttime independent of any other timing unit. However the probability ofthis occurring is acceptable since only a few transmitters are likely tobe re-activated at any one time and it is likely that the opticalradiation power level not to exceed the set safe level.

In an alternative arrangement the timing unit generates a restart timebased on the wavelength channel.

Another possibility is that an operator sets restart times for thedifferent wavelength channels, for example, at the time that the clientequipment is installed.

A transmitter system in accordance with the invention is especiallysuited to a passive WDM system which includes little or no controlbetween the wavelength channels (client equipment) and which includespassive (optical) components for performing the WDM functions (inparticular optical multiplexing of the WDM wavelength channels). Howevera transmitter/receiver system in accordance with the invention using astaggered re-start of lasers after interruption is also considered to beadvantageous in other WDM system.

1-9. (canceled) 10: An optical transmitter system, comprising: aplurality of transmitters for supplying a plurality of optical radiationcomponents having respective wavelengths to a transmission elementoperative for conveying the plurality of radiation components to arespective receiver; an interrupter for interrupting the supply of theradiation components; a timer for providing a respective restart timewhich differ one to another for each of the radiation components; and acontroller for restarting the supply of the radiation components,following an interruption, at the respective restart time provided bythe timer. 11: The transmitter system according to claim 10, in whichthe timer is operative for generating a pseudo-random sequence ofrespective restart times. 12: The transmitter system according to claim10, in which the timer includes a respective timing unit at eachtransmitter which is capable of providing the respective restart time.13: The transmitter system according to claim 12, in which the timingunit at each transmitter includes a pseudo-random number generatorhaving a seed number used in the pseudo-random number generator which isunique to each transmitter. 14: The transmitter system according toclaim 10, in which the timer is operative for providing the respectiverestart times based on the wavelengths of the radiation components. 15:The transmitter system according to claim 10, in which the timer isoperative for providing the respective restart times which are preset ata time of installation of the transmitter system. 16: The transmittersystem according to claim 13, in which the seed number used in thepseudo-random number generator includes a network address and a portnumber of each transmitter. 17: The transmitter system according toclaim 13, in which the seed number used in the pseudo-random numbergenerator includes a laser card serial number of the respectivetransmitter. 18: The transmitter system according to claim 10, in whichthe interrupter is operative for interrupting the supply of theradiation components in response to a loss of a radiation component atthe respective receiver. 19: A method of operating an opticaltransmitter system, comprising the steps of: supplying a plurality ofdifferent wavelength optical radiation components to a transmissionelement for transmission to a respective receiver; interrupting a supplyof the wavelength radiation components; and supplying respective restarttimes which differ one to another for the different wavelength radiationcomponents and, following an interruption, restarting the supply of thewavelength radiation components at the respective restart time. 20: Themethod according to claim 19, in which the interrupting step isperformed by interrupting the supply of the radiation components inresponse to a loss of a radiation component at the respective receiver.